Polymerizable, liquid-crystalline polysiloxanes and processes for their preparation are disclosed, for example, in U.S. Pat. No. 4,388,453 and U.S. Pat. No. 5,211,877. However, on polymerization the polysiloxanes described therein form polymers which are three-dimensionally crosslinked and are therefore brittle and fracture easily. Furthermore, the polysiloxanes described therein have high viscosity, i.e., can only be processed at elevated temperatures, which in turn can result in premature partial polymerization of the materials. At lower temperatures, the establishment of equilibrium states, such as alignment, is hindered by the high viscosity.
Owing to the preparation processes described therein, the oligomeric and polymeric compounds described are not defined homogeneous compounds, but instead are always statistical mixtures of siloxanes having different substitution patterns. This can result in nonhomogeneous or heterogeneous phases, which cannot be used for optical applications. Furthermore, these mixtures generally cannot be polymerized fully, since non-polymerizable siloxanes are present in various amounts. These mixtures generally cannot be crystallized, which makes simple purification of the siloxanes more difficult.
The known polymerizable liquid-crystalline polysiloxanes, if they have a cholesteric phase, have only low temperature dependence of the color of the cholesteric reflection within the temperature range available for processing, i.e., they exhibit only weak thermochromic properties. For use in photolithographically structurable filters, however, pronounced thermochromic properties are an absolute necessity.
Polymerizable polysiloxanes containing mesogenic groups are also disclosed in EP-A-471 277. However, the monomeric siloxanes described therein usually only form crystalline or liquid phases. Only in a few cases are highly ordered liquid-crystalline phases obtained. However, highly ordered liquid-crystalline phases are distinguished by high viscosity and can therefore only be processed in solution or the melt with loss of their liquid-crystalline property and/or at high temperatures, which can result in premature polymerization of the materials. Furthermore, highly ordered phases cannot be applied and aligned by simple methods such as knife coating or spin coating. In addition, highly ordered phases do not have colored reflection and therefore do not have thermochromic properties either.